The narcissistic mother is a control freak and does not easily relinquish good and reliable sources of "narcissistic supply" (admiration, adulation, attention of any kind). It is the role of her children to replenish this supply, the children owe it to her. To make sure that the child does not develop boundaries, and does not become independent, or autonomous, the narcissistic parent micromanages the child's life and encourages dependent and infantile behaviors in her offspring.
Such a parent bribes the child (by offering free lodging or financial support or "help" with daily tasks) or emotionally blackmails the child (by constantly demanding help and imposing chores, claiming to be ill or disabled) or even threatens the child (for instance: to disinherit her if she does not comply with the parent's wishes). The narcissistic mother also does her best to scare away anyone who may upset this symbiotic relationship or otherwise threaten the delicate, unspoken contract. She sabotages any budding relationship her child develops with lies, deceit, and scorn.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase: http://www.narcissistic-abuse.com/thebook.html)